"When it comes to tennis, good or bad - there's really only one thing that I know for certain - I've missed it," Sharapova wrote on the Players' Tribune website.

"Though these last two years have been tougher - so much tougher - than I ever could have anticipated... my passion for the game has never wavered. If anything, it has only grown stronger."

Sharapova was issued a two-year suspension after testing positive for the banned heart and blood boosting drug meldonium at the 2016 Australian Open, but the Court of Arbitration for Sport reduced the ban on appeal.

She said that she had taken it for several years and did not know it had been placed on the banned list at the start of 2016.

Sharapova made her return in April at Stuttgart, reaching the semi-finals as a wildcard, but such invitations sparked criticism from some WTA rivals, saying she should have to work her way back without such benefits, some suggesting a life ban.

Shaking off criticism after the French Open snub, Sharapova tweeted, "If this is what it takes to rise up again, then I am in it all the way, everyday. No words, games or actions will ever stop me from reaching my own dreams. And I have many."

Sharapova has missed three of the past four US Opens, her last visit ending in the 2014 fourth round.

The USTA defended its wildcard offer, saying, "Her suspension under the terms of the tennis anti-doping program was completed and therefore was not one of the factors weighed in our wildcard selection process."

"Consistent with past practice, a wildcard was provided to a past US champion who needed the wildcard for entry into the main draw."

Former world No 1 Sharapova won the 2006 US Open as well as Wimbledon in 2004, the 2008 Australian Open and the 2012 and 2014 French Opens.

"I'm sure this is a great moment for her to have an opportunity to be back to a place where she has done so well," seven-time major champion Venus Williams said.

Others expressed support, too

Glad to see USTA give Maria Sharapova a wild card, suspension over, great for the sport to have her back. https://t.co/0l0jJuMKfL